1. Field of the Invention
The present invention relates to an ophthalmic measurement apparatus, and more particularly relates to an ophthalmic measurement apparatus such as an eye refractive power measurement apparatus for measuring a refractive power of an eye to be examined objectively.
2. Description of Related Art
In the case that the refractive error is corrected by using spectacles or contact lens, a subjective inspection is performed in order to determine a prescribed value thereof. Upon performing the subjective inspection, such method is widely spread that uses measured data which are measured by an eye refractive power measurement apparatus for measuring a refractive power of an eye to be examined objectively. It is known an eye refractive power measurement apparatus that scans slit-shape light bundle and projects it onto a fundus of the eye, and obtains the refractive power of the eye by detecting the light reflected by the fundus of the eye due to the projection of slit light bundles by means of two pairs of photo-receiving elements which are disposed so as to be symmetric with putting an optical axis therebetween at approximately conjugate position relative to a cornea of the eye. The results measured by the apparatus are calculated and outputted by using three parameters S (spherical power), C (astigmatism (cylindrical) power) and A (astigmatism (cylindrical) axial angle), with defining that the refractive power of the eye is symmetric with respect to the corneal center in order to fit to the prescribed value of spectacles or the like.
However, a refractive power of an eye is not always symmetric with respect to the corneal center, and many eyes have irregular astigmatism. In the case of irregular astigmatism such as keratoconus, there is difference between the measured results of S, C and A obtained at the time when the eye looks at the center of the fixation target inside of the apparatus and the measured results of S, C and A obtained at the time when the eye looks at the point except the center of the fixation target. Therefore, measured data of conventional art does not provide sufficient refractive information enough to perform the subjective inspection efficiently.
Also, recently, the keratorefractive surgery which corrects the refractive error by varying the corneal curvature artificially by removing the corneal surface is noticeable, therefore such apparatus is required that confirms the corneal shape in detail at pre-operation and post-operation, and that recognizes the distribution of the refractive power at respective parts of the cornea. The reason for such requirements, the finally purpose of the keratorefractive surgery resides in to make the distribution of the refractive power of the eye be close to emmetropia (or low myopia, low regular astigmatism).
Further, referring to the keratorefractive surgery by using excimer laser beam which is widely known as recent correcting method for refractive error, the corneal curvature is varied by removing the corneal surface, once there exists the axial deviation, the symmetric performance of the cornea is broken. In addition, referring to such an eye to be examined that is not retained symmetric performance of the refractive power as described above, the measured results of S, C and A often tend to be improper. In this case, the inspection performed for spectacles prescription and the like based on the subjective inspection continued thereafter takes much time, therefore the eye is made to be tired, and it often results in making improper prescribed value.